1. Field of the Invention
This invention relates to a system camera provided with a rear conversion lens barrel interposed between an interchangeable photo-taking lens barrel and a camera body.
2. Related Background Art
As a focus detecting apparatus in a TTL camera, there is known an automatic focus detecting apparatus of the so-called pupil division type which detects the focus adjusted state of a photo-taking optical system from the amount of relative deviation of a plurality of object images formed by light beams coming from different areas of the pupil of the photo-taking optical system.
For example, Japanese Patent Publication No. 57-49841 discloses an automatic focus detecting apparatus of this type which is constituted by a pair of arrays, i.e., a lens array disposed near the primary image plane and a light receiving element array disposed immediately rearwardly thereof.
Also, Japanese Laid-Open Patent Application No 54-104859 discloses an automatic focus detecting apparatus of this type which is constituted by a field lens disposed on the primary image plane, two re-imaging lenses for re-imaging an image formed on the primary image plane on the secondary image plane, and two image sensor arrays disposed on the secondary image plate.
However, such automatic focus detecting apparatuses of the pupil division type according to the prior art have suffered from the disadvantage as will hereinafter be described.
In the automatic focus detecting apparatus of this type, the pupil of a predetermined F number is specified on a place orthogonal to a predetermined position on the optical axis by an optical system on the focus detection side and therefore, even if the F number of the exit pupil is an F number great as compared with a predetermined F number, or the F number of the exit pupil is the same F number as said predetermined F number, or is an F number small as compared with said predetermined F number, when an interchangeable photo-taking lens in which the position of the exit pupil from said predetermined position is mounted on a camera body having said automatic focus detecting apparatus, eclipse may sometimes be caused in a focus detecting light beam by the position of the exit pupil, and when the eclipse becomes non-uniform on the image plane on the focus detecting optical system side, distortion is formed in the balance between the quantities of light of the object images and thus, it has been impossible to accomplish accurate focus detection.
To overcome such a disadvantage, Japanese Laid-Open Patent Application No. 60-86517 discloses a focus detecting apparatus provided with eclipsed state detecting means for detecting the eclipsed state of an image formed by a focus detecting optical system, by a pair of outputs put out from an image sensor, and outputting a signal corresponding to that state.
In such a system, the eclipsed state is directly found from the outputs of the image sensor and therefore, the eclipsed state can be detected highly accurately for objects which satisfy special conditions as when the objects are uniformly approximate to intensity of illumination.
However, when an object has a complicated luminance distribution, the pattern of the outputs of the image sensor is also complicated and moreover, the image position regarding the pair of image outputs differs depending on the defocus amount and therefore, the eclipsed state has not always been detected accurately even if various contrivances have been made.
So, in Japanese Laid-Open Patent Application No. 62-229213, attention is paid to the relation between eclipse and the position of the exit pupil of a photo-taking optical system, and information regarding said exit pupil is stored in memory means contained in an interchangeable photo-taking lens, and this information is read on the body side as required and the amount of eclipse is estimated, whereby an accurate amount of eclipse is grasped irrespective of objects to thereby enhance the accuracy of focus detection.
Also, as a countermeasure for any exposure error caused by a lens mounted on a lens interchange type camera in which light from an object incident through a photo-taking lens is received by the light receiving portion of an exposure meter and the degree of exposure is measured, Japanese Patent Publication No. 41-12429, Japanese Laid-Open Patent Application No. 62-86337 and Japanese Laid-Open Patent Application No. 1-109331 make it possible to transmit the information of the position of the exit pupil of the interchangeable lens to the camera body and correct exposure to thereby obtain accurate exposure.
On the other hand, there is also known a focus detecting apparatus in which a defocus amount output from defocus amount detecting means is corrected in conformity with the spherical aberration of an imaging optical system and the best focus position is determined.
For example, Japanese Laid-Open Patent Application No. 57-210326 discloses a system in which a signal for identifying the type of a photo-taking lens provided in an interchangeable lens barrel is read on the camera body side, whereby the type of spherical aberration is identified and correction is effected by corresponding spherical aberration data. This intends to correct the amount of out-of-focus with the F-value used in photographing being also taken into account.
In this case, however, a memory device for making the amount of correction of the best image plane position to correspond from the types of aberrations and the F-value used in photographing is contained in the camera body and when an attempt is made to effect fine correction, the types of aberrations differ from one photo-taking lens to another and also, in a zoom lens, the types of aberrations differ in the zoom position and therefore, the number of types becomes huge, and this has led to the disadvantage that an amount of memory which cannot be coped with by only the memory device in the camera body in both of cost and capacity becomes necessary.
There has also been the disadvantage that the types of the aberrations of a photo-taking lens developed after the manufacture of the camera body cannot be coped with if they do not belong to any former types.
A focus detecting apparatus of this type is also disclosed in Japanese Laid-Open Patent Application No 59-208514.
This has the merit that whatever types of aberrations of a photo-taking lens developed after the sale of the camera body can be coped with, while the amount of correction contained in the photo-taking lens side is determined on the premise that a focus detecting apparatus having a particular aperture-corresponding F-value is contained in the camera body. Accordingly, this has led to the disadvantage that where the aperture-corresponding F-value of a focus detecting apparatus developed later differs from that of a formerly developed focus detecting apparatus, the amount of correction contained in the lens cannot be applied to such detecting apparatus.
There has also been the disadvantage that a case where the focus detecting apparatus contained in the camera body has a plurality of aperture-corresponding F-values cannot be coped with.
The focus detecting apparatus disclosed in the aforementioned Japanese Laid-Open Patent Application No. 57-210326 is free from such disadvantages, because the memory means for the amount of correction is on the body side and when the aperture-corresponding F-value of the focus detecting apparatus contained in the camera body changes, the content of the memory means can be changed correspondingly thereto.
The aberration correction system of the above-described focus detecting apparatus also has the disadvantage that it cannot cope with one of a lens which will be put on sale in the future and a camera body containing therein a focus detecting apparatus which will be developed in the future.
To overcome these disadvantages, Japanese Laid-Open Patent Application No. 62-227108 provides a correction system which can correct aberrations by providing in a removably mountable photo-taking lens a means for storing therein two or more independent data expressive of the characteristics of the aberrations of the photo-taking lens whatever may be a lens which will be developed in the future or a focus detecting apparatus contained in a camera body which will be developed in the future.
As will be seen from the above-mentioned publications, the information regarding the exit pupil position and aberrations of the photo-taking lens is indispensable in accurate focus detection, and the information regarding the exit pupil position of the photo-taking lens is indispensable in accurate exposure.
Now, in a system camera provided with the focus detecting apparatus disclosed in the aforementioned Japanese Laid-Open Patent Application No. 62-229213 or Japanese Laid-Open Patent Application No. 62-227108, where a rear conversion lens is interposed between an interchangeable photo-taking lens barrel and a camera body, information regarding only the interchangeable lens is read on the body side in spite of the fact that data required by the camera body side is information regarding the exit pupil position and aberrations in a state in which the interchangeable photo-taking lens barrel and the rear conversion lens are made into a single optical system, and this has led to the problem that desired information cannot be obtained and this information difference provides an error in focus detection and the accuracy of focus detection is remarkably reduced.
The system cameras disclosed in Japanese Patent Publication No. 41-12429, Japanese Laid-Open Patent Application No. 62-86337 and Japanese Laid-Open Patent Application No. 1-109331 have also suffered from the problem that exposure accuracy is likewise remarkably reduced by information difference.
Further, in the system cameras according to the prior art, with the interchangeable photo-taking lens and the rear conversion lens as a single optical system, the necessity of storing all information regarding the combination of various interchangeable photo-taking lenses and the rear conversion lens has arisen, and this has led to the problem that data memory capacity becomes great and cost increases.